1. Field of the Invention
The present invention relates to a method for reconstructing an image subject from image data obtained by magnetic resonance.
2. Description of the Prior Art
An image reconstruction method is known from xe2x80x9cPhase Alignment of Multiple Surface Coil Data for Reduced Bandwidth and Reconstruction Requirements,xe2x80x9d published in Magnetic Resonance in Medicine, vol. 38 (1997): 1003-1011, wherein a subject is excited to a state of magnetic resonance in gradient fields of different spatially encoded frequencies in succession, and for each frequency an MR signal of the subject is received by at least two MR antennas, the totality of the received MR signals forming an MR spectrum, from which an image of the subject is reconstructed, and wherein the MR signals received by one of the MR antennas has a phase angle relative to the MR signals received by the other MR antenna, and wherein this phase relation is determined by a computing unit, and for each frequency the MR signals of the two MR antennas are added as phase-corrected signals to form a sum signal, and this sum signal is used in the further reconstruction of the image of the subject.
This known image reconstruction method already functions rather well. In particular, it has a good signal-to-noise ratio and requires only one Fourier transform for the image reconstruction. In this known method, the phase relation is determined using a preliminary scan. Later modifications are no longer possible, or are possible only with degradation of the signal-to-noise ratio.
An object of the present invention is to further develop the known image reconstruction method such that the current phase relation of the MR signals to one another can be determined at any time.
The object is achieved in accordance with the invention in an image reconstruction method of the type described above, wherein the phase relation is determined bythe computing unit automatically from a correlation of the MR signals that are received by the MR antenna.
It is possible in this way to readjust the MR antennas at any time. In particular, the possibility is created to adjust the geometric position of the MR antennas even during the examination of a subject. It is thus possible to pick up at least one first MR spectrum and one second MR spectrum in succession, at least one of the MR antennas being arranged differently in relation to the subject during the reception of the second MR spectrum than during the reception of the first MR spectrum.
When the received MR signals are digitized, and the phase correction of the MR signals is performed subsequent to the digitizing of the MR signals received by the MR antennas, the phase correction of the MR signals is particularly simple to accomplish. It is not necessary to execute the correlation process, owing to simultaneously received MR signals. The MR signals also can be received by the antennas with a short time separation.
Alternatively, the phase correction can also be preformed priorto the digitization of the MR signals.
The phase correction is even simpler (particularly when it precedes the digitization process) when at least one of the MR signals is shifted in relation to another of the MR signals by a whole-number multiple of a step angle, so that the phase relation of the shifted MR signal to the other MR signal is as at most half the step angle. The step angle can be between 10xc2x0 and 20xc2x0, for instance 15xc2x0.
The determination of the correlation is also sufficiently precise when it is performed in correlation steps with a stepwidth between 10xc2x0 and 20xc2x0, for example.